Review

While we aren't aware of any published research on the advantages of
unidirecdtional tunnels,
they appear to make it harder to detect a
request/response pattern, which is quite possible to detect over a
bidirectional tunnel.
Several apps and protocols, notably HTTP,
do transfer data in such manner. Having the traffic follow the same
route to its destination and back could make it easier for an
attacker who has only timing and traffic volume data to infer the path a
tunnel is taking.
Having the response come back along a different path arguably
makes it harder.

When dealing with
an internal adversary or most external adversaries, I2P's undirectional tunnels
expose half as much traffic data than would be exposed with bidirectional circuits
by simply looking at the flows themselves - an HTTP request and response would
follow the same path in Tor, while in I2P the packets making up the request
would go out through one or more outbound tunnels and the packets making up
the response would come back through one or more different inbound tunnels.

The strategy of using two separate tunnels for inbound and outbound
communication is not the only technique available, and it does have anonymity
implications. On the positive side, by using separate tunnels it lessens the
traffic data exposed for analysis to participants in a tunnel - for instance,
peers in an outbound tunnel from a web browser would only see the traffic of
an HTTP GET, while the peers in an inbound tunnel would see the payload
delivered along the tunnel. With bidirectional tunnels, all participants would
have access to the fact that e.g. 1KB was sent in one direction, then 100KB
in the other. On the negative side, using unidirectional tunnels means that
there are two sets of peers which need to be profiled and accounted for, and
additional care must be taken to address the increased speed of predecessor
attacks. The tunnel pooling and building process
(peer selection and ordering strategies)
should minimize the worries of the predecessor attack.

Anonymity

The paper's main point is that
deanonymizations on unidirectional tunnels take a longer time, which is an
advantage, but that an attacker can be more certain in the unidirectional case.
Therefore, the paper claims it isn't an advantage at all, but a disadvantage, at least
with long-living eepsites.

This conclusion is not fully supported by the paper. Unidirectional tunnels clearly
mitigate other attacks and it's not clear how to trade off the risk of the
attack in the paper
with attacks on a bidirectional tunnel architecture.

This conclusion is based on an arbitrary certainty vs. time weighting
(tradeoff) that may not be applicable in all cases. For
example, somebody could make a list of possible IPs then issue subpoenas to
each. Or the attacker could DDoS each in turn and via a simple
intersection attack see if the eepsite goes down or is slowed down. So close
may be good enough, or time may be more important.

The conclusion is based on a specific weighting of the importance of certainty
vs. time, and that weighting may be wrong, and it's definitely debatable,
especially in a real world with subpoenas, search warrants, and other methods
available for final confirmation.

A full analysis of the tradeoffs of unidirectional vs. bidirectional
tunnels is clearly outside the scope of the paper, and has not been done
elsewhere. For example, how does this attack compare to the numerous possible
timing attacks published about onion-routed networks? Clearly the authors have not
done that analysis, if it's even possible to do it
effectively.

Tor uses bidirectional tunnels and has had a lot of academic review. I2P
uses unidirectional tunnels and has had very little review. Does the lack of a
research paper defending unidirectional tunnels mean that it is a poor design
choice, or just that it needs more study? Timing attacks and
distributed attacks are difficult to defend against in both I2P and Tor. The
design intent (see references above) was that unidirectional tunnels are more
resistant to timing attacks. However, the paper presents a somewhat different type of timing
attack. Is this attack, innovative as it is, sufficient to label I2P's
tunnel architecture (and thus I2P as a whole) a "bad design", and by
implication clearly inferior to Tor, or is it just a design alternative that
clearly needs further investigation and analysis? There are several other reasons
to consider I2P currently inferior to Tor and other projects (small network
size, lack of funding, lack of review) but is unidirectional tunnels really a
reason?

In summary, "bad design decision" is apparently (since the paper does
not label bidirectional tunnels "bad") shorthand for "unidirectional
tunnels are unequivocally inferior to bidirectional tunnels", yet this
conclusion is not supported by the paper.